Possible Zika vaccines are at least 18 months away from large-scale trials, according to the World Health Organisation.

WHO assistant director-general for health systems and innovation Marie-Paule Kieny said the UN health agency's response is "proceeding very quickly" and 15 companies or groups have been identified as possible participants in the hunt for vaccines.

She told reporters in Geneva that the WHO also believes the link between the mosquito-borne virus and abnormally small heads in some newborn children is "more and more probable".

The Zika outbreak is spreading rapidly across Latin America.

"Our knowledge of what is currently in the pipeline tells us that it will take approximately 18 months before a vaccine can be launched into large-scale trial to demonstrate efficacy," Ms Kieny said.

Concerns have grown in recent months about a Zika outbreak that has affected at least 33 countries - mostly in South and Central America. In Brazil, an epicentre of the outbreak, the spike in cases has coincided with a mysterious rise in cases of microcephaly, or abnormally small heads, in newborn children.

The WHO believes the link between the virus and microcephaly in some newborns is "more and more probable", Mr Kieny said, but it will take "weeks to a few months" to determine whether a firm link exists.

Agency officials had previously said they believed it would take at least six months.

Ms Kieny said different types of possible vaccines - such as live or killed virus, or use of DNA vaccines - may lead to differences in timing on development, but developers "are all starting at a very basic level for the time being".

The WHO also issued updated guidelines about travel to Zika-affected areas, saying pregnant women should consult health care providers and "consider delaying travel to any area where locally acquired Zika infection is occurring".

It previously only advised women who are pregnant, or hoping to become pregnant, to protect themselves from the bites of the mosquito that transmits the virus.

In a statement, the agency noted that the virus was not spread by person-to-person contact, "though a small number of cases of sexual transmission have been documented".

The WHO also advised men and women returning from Zika-affected areas to practise safe sex "including through the correct and consistent use of condoms", but did not specify for how long.

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